It is well known that electroluminescence is exhibited in the vicinity of a PN junction which is biased so as to inject charge carriers of one type into a region where the predominant charge carriers are of opposite type. Light is emitted in conjunction with the re-combination of pairs of oppositely charged carriers.
Electroluminescent diodes are generally formed of single crystal wafers of group III-V materials, such as gallium arsenide and gallium phosphide, having a PN junction therein. The electroluminescent light that is generated by the recombination of pairs of oppositely charged carriers in the single crystal wafers has great difficulty escaping the crystal. Since the crystals have high indices of refraction, generally about 3.5, and are usually in the shape of rectangular parallelepipeds internal total reflection permits only light photons emitted within a small angle to be transmitted through the surface. This is only a few percent of the total emitted light. The rest of the light is totally reflected from surface to surface until it is finally absorbed inside the crystal or until it escapes via an irregularity in the surface of the crystal.
Attempts have been made to overcome the loss mechanism. One method used has been to shape the crystal in the form of a hemisphere with the light-emitting junction located at the flat bottom surface of the hemisphere. Although this construction has achieved a substantial increase in the amount of emitted light, its fabrication has a number of disadvantages. In one fabrication method two diode cubes of material from the group III-V materials are placed back to back and are tumbled in an abrasive material until the desired hemispherical shape is achieved at which time the spheres are separated. In another method a glass dome is formed over an electroluminescent diode. The glass dome may be formed by placing a preformed glass bead on a heated diode and support subassembly, or by melting a glass in a mold cavity and placing the diode and support subassembly onto the soft glass while in the mold as described in U.S. Pat. No. 3,596,136. The latter method does not overcome the problem of total internal reflection since the diode still has the form of a parallelpiped. However, the angle of escape is somewhat increased. Both of the methods described are costly and time consuming and therefore not suited to large scale production. Also, the described methods cannot produce an entire array of light emitting diodes supported on a common substrate.